Added methods to surface number of zone and scoped handles for each isolate.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 8304532470999c81ea515843ad6496c8348db0f3..963b212bf687e788381d66e650b2a993b553bfa4 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -2044,6 +2044,12 @@ void Isolate::PrintJSON(JSONStream* stream, bool ref) {
     jsobj.AddProperty("rootLib", lib);
   }
 
+  intptr_t zone_handle_count = thread_registry_->CountZoneHandles();
+  intptr_t scoped_handle_count = thread_registry_->CountScopedHandles();
+
+  jsobj.AddProperty("_numZoneHandles", zone_handle_count);
+  jsobj.AddProperty("_numScopedHandles", scoped_handle_count);
+
   if (FLAG_profiler) {
     JSONObject tagCounters(&jsobj, "_tagCounters");
     vm_tag_counters()->PrintToJSONObject(&tagCounters);